High B+

[dpierce05]

Hey everyone im new to this forum, but i just finished a project that i got off of the Hoffman site. Its called the schedule 40 and is based on a Gibson GA5. Instead of a 6V6 it uses a EL84, the voltages on the EL84 seem to be really high to me. I'm getting 400VDC on pin 9 and 386VDC on pin 7, I'm running a 5V4GA rectifier its a tung-sol NOS, JJEL84 for the power and a EH12AX7 in the pre-amp. The power transformer I used is a weber Champ Princeton and has 2 different sets of high voltage taps 660 and 600, I used the 660 taps. I used a Allem amps choke and a Mojo Output transformer for a Champ with 4 and 8 OHM Taps any ideas on how to get the B+ down? I've read that in this set-up the EL84 should probably see about 300 to 350 VDC any ideas or suggestions would be appreciated.

[MWJB]

When you rectify that 660VAC or 600VAC to dc you get more than the original VAC in dc at the B+ rail. E.g. 660VAC will give 330VAC from each leg of the B+ secondary, typically a 5V4 will multiply that 330VAC by a factor of 1.25 = 412vdc, less whatever voltage that the current draw of the power tube pulls down slightly. So for the parts that you have used & the way you have them wired, everything is behaving like it should. 125P1B transformers typically kick out a little more than 330VAC from each leg of the B+ secondary, so your amp would appear to be running in the ball park for the Shedule 40 design. It does not look to me as if they had any intention of running the EL84 at 300-350vdc.

As even a NOS 5Y3 will multiply up that AC voltage some (factor of ~1.1) you can see that you are simply not going to get much less than 330vdc after the rectifier if you switch to the 300-0-300VAC taps & follow the schematic. You have 3 options really...

Switch to a NOS 5Y3 - you'll still be over 300v. The Epi Valve Jr runs these kinds of voltages but most classic EL84 SE amps run nearer 250vdc. You may want to try a 220ohm cathode resistor with this route.

Do the above but relocate your choke (assuming that it is rated for the full current draw on the B+ rail - check with D. Allen) to before the first filter cap & add another 22uf cap at the upstream end (16uf would work too). You could relocate the cap that is currently located at the screen grid resistor. This will pull down the voltage a little more. Again rebiasing the EL84 may be necessary.

Leave everything as it is, just replace your 9 pin socket with an octal socket, use a 6V6 power tube with a 470ohm cathode resistor (5W rating).

...or leave everything as it is and see how you go, if you like the sound then all well & good, even IF it eats EL84s evey year/several months then they're cheap and it won't break the bank. There is a Russian Sovtek EL84M/E84L that will take the higher voltages.

[dpierce05]

Thank You so much for your help. I love the way the amp sounds as it is I think I'll try the 5Y3 route and see what happens. Again Thank you for your help.